In composing a television production, there is an ever increasing need for variety of special effects. The phrase, special effects, is used to broadly define the selective altering and/or manipulating of video data to create a visual effect to attract the attention of the viewer. Special effects are used in a variety of video production situations.
There are a number of devices that create these special effects. Video switchers are one device that can produce special effects. The special effects produced by a video switcher are most commonly used while switching between video sources. Common switcher special effects include dissolves and wipes. An example of a video switcher that can produce a variety of dissolves and wipes is the AVC video switcher of Ampex Corporation.
Another device for producing special effects is the digital special effects system, such as the ADO digital special effects system of Ampex Corporation. Digital special effects systems can perform a number of special effects such as video frame enlargement and reduction, frame movement, frame rotation in 2 and 3 dimensions, and perspective manipulation. Each of these effects can be combined with other special effects to form even more complex effects.
The output from a digital special effects system commonly includes not only an output video signal, but also a corresponding key signal. The key signal indicates the level of the video signal, that is the ratio of the gain of the video signal relative to its original gain in the corresponding video signal. Key signals can be created by a variety of video devices.
There are two types of key signals. The first is a bi-state key signal, which simply indicates whether the corresponding part of the video signal is to be retained. If the corresponding part of the video signal is to be retained, the key signal has a value of one, and if it is not to be retained, then the key signal has a value of zero.
A more generally useful type of key signal is a linear key signal. Instead of having just two possible values, the linear key can have any value from zero to one. Thus, the value of a linear key can be used to partially reduce the level of a video signal. When viewed on a video monitor, a corresponding video image would exhibit a level of transparency corresponding to the value of the key signal.
The linear key is one method to allow two video signals to be added. If two video signals were simply added, their combined dynamic range would be twice the amount that could be handled by the video system. In order to add two video signals, their dynamic range must be reduced such that when added, their total dynamic range is equal to the maximum dynamic range or less. Processing the video signals with linear keys is one method to effect this dynamic range reduction.
A key signal is used to process a video signal. A video signal is processed by a key signal by multiplying the key signal with the video signal. Parts of a video signal which are processed with a corresponding part of a key signal which has a value of zero, will result in keyed video signal that has a signal level of zero. Conversely, when processed with a key signal of one, the keyed video signal will be left unchanged. When processed with some intermediate value of a linear key, the signal level of the video signal is reduced by that amount, effectively resulting in the video signal appearing transparent, if viewed on a video monitor.
Two or more keyed video signals can be combined only they have been processed by key signals totaling one or less. For example, if a first keyed video signal has been processed by a key signal of 0.5 and a second keyed video signal has been processed by a key of 0.4, the two keyed video signals could be combined, because the total of the key signals is 0.9, which is less than one. However, a video signal processed by a key of 0.5 cannot be combined with a video signal processed by a key of 0.6, because the total of the key signals would be 1.1, which is greater than one. In practice, a decision is not usually made on whether to combine video signals based on their associated keys, but rather key signals are adjusted so as to allow the combining of video signals.
An unfortunate problem associated with the sale of special effects machines is that the more often a particular effect is used, its novelty disappears and thus its ability to attract attention diminishes. Thus there is a constant need for new effects. Additionally, the larger the library of potential effects available to the operator, the less often the operator will need to call upon a particular effect. There is a need, therefore, for a variety of special effects.
Framestores have been used in the video industry for many years. A framestore is a memory device that can store one complete frame of video data. Framestores are commonly digital. Digital framestores allow manipulation of stored video data. Digital framestores are commonly used in digital special effects systems.
Framestores are typically configured to input a frame of video from one part of a system and output the frame to another part of the system. When a new frame of video is inputted to the framestore, the previous frame is completely overwritten.
A frame of video is actually composed of two interlaced fields of video. A frame of video contains a certain number of horizontal lines of video information (for example, 525 lines in the NTSC system used in the United States). Each field is composed of every other line which are interlaced to form a frame of video data. A frame of video is recorded by first recording one the fields and successively recording the other field. A typical time period between the recording of successive fields is one-sixtieth of a second (in NTSC system). If there is significant movement in the images of the video signal, there will be an annoying flicker when two fields are viewed as a video still frame. This can be a problem when such fields are combined in a framestore. One method to solve this problem is to use only a process called alternate field interpolation. Alternate field interpolation uses only one field of video from each frame and interpolates the second field from the first field. Because the second field is based entirely on the first field, there is no movement between fields in a frame. The disadvantage of this method is that one half of the video information is being thrown away. When there is little or no movement between fields, the entire frame should be used. Therefore, there is a need for a method and apparatus to prevent flicker when there is movement between two fields of a frame of video that makes use of the entire original frame.
It would be advantageous if the output of a framestore could be recirculated back through the framestore with the addition of new input frames. This would allow the combining of many frames of video to form an output signal. A number of special effects can be created by this recirculation, including blurs, smears, and trails, all highly desirable special effects. Such recirculation requires the ability to combine video signals. This ability to combine video signals requires the ability to manipulate video signals and associated key signals. Such a framestore would require key signal processing, storage and recirculating capability.
Another desirable special effect would be the ability to decay or selectively remove recirculated frames. Recirculated frames can be decayed in a variety of ways, including decaying in a preset order or in a random pattern. If decayed in some preset order, a method needs to be provided to determine the order of decay. If removed randomly, a method of generating the random decay needs to be provided.
While such a framestore could desirably be operated as part of many video systems, such as a switcher or digital special effects system, it could also be operated as a stand alone unit. A particularly desirable use is in conjunction with a video combiner or concentrator. A video concentrator is a device which selectively combines various video signals and their associated key signals to form one or more output video signals. An example of such a video concentrator, is the ADO Concentrator of Ampex Corporation. Because of the variety of input signals available, the video concentrator is an ideal location for a special effects frame store.